1. Field of the Invention
The present invention relates to novel electrode structures which are of use in electrochemical devices, particularly fuel cells, and it teaches new processes for the manufacture of electrode and stack designs. The invention improves also alkaline direct methanol fuel cells.
2. Description of Related Art
Electrochemical cells invariably comprise at their fundamental level a solid or liquid electrolyte and two electrodes, the anode and cathode, at which the desired electrochemical reactions take place. Porous electrodes are employed in a range of electrochemical devices, in which a gaseous or liquid reactant and/or product has to be transferred into or out of one of the cell electrode structures. They are designed to optimise the contact between the reactant and the electrolyte to maximise the reaction rate. Catalysts are often incorporated into electrode structures to increase the rates of the desired electrode reactions.
Porous metal or carbon electrodes are employed in many different electrochemical devices, including metal-air batteries, electrochemical gas sensors, electrosynthesis of useful chemical compounds, and in particular, fuel cells.
A fuel cell is an energy conversion device that efficiently converts the stored chemical energy of its fuel into electrical energy by combining either hydrogen, stored as a gas, or methanol stored as a liquid or gas, with oxygen to generate electrical power. The hydrogen or methanol are oxidised at the anode and oxygen (or air) is reduced at the cathode. Both electrodes are of the porous type. The electrolyte has to be in contact with both electrodes and may be acidic or alkaline, liquid, solid or a membrane. The electrodes are designed to be porous and allow the reactant hydrogen or methanol to enter the electrode from the face of the electrode exposed to the reactant fuel supply, and diffuse through the thickness of the electrode to the reaction sites which contain catalysts, usually platinum metal based, to maximise the electrochemical oxidation of hydrogen or methanol. The anode is designed to be surface-wetted by the electrolyte to contact the same reaction sites. With alkaline electrolyte types the product of the hydrogen reaction is water. The water transpires through the porous electrode into the gas space behind the anode. The cathode is also designed to be porous and allow oxygen or air to enter the electrode and diffuse through to the reaction sites. Catalysts are again commonly incorporated to maximise the rate of the oxygen reaction (peroxide-mechanism) at the cathode reaction sites. The reaction of the methanol on the anode produces carbon dioxide, which forms carbonate with the caustic electrolyte. When methanol is exhausted, the electrolyte is exchanged. When the cell is re-fueled, a mix of methanol and caustic must be used. This is only possible with an exchangeable or circulating electrolyte system.
The porous electrodes of Fuel Cells comprise many components and are typically made up of one or more layers. Typically the porous electrode will comprise one or more catalyst containing layers, which are supported onto a more rigid porous substrate layer. The catalyst containing layers enhance the desired electrode reactions and comprise a catalyst, which may be formed on a high surface carbon material. Catalysts are often precious metals, particularly platinum alloys in a very high surface area form, dispersed and supported on a high surface area electrically conducting porous carbon, black or graphite (for example U.S. Pat. No. 4,447,505). The catalyst component may also be a non precious metal, such as one of the transition metals. In fuel cells which employ alkaline electrolytes, the cathode can comprise catalysts based on macrocyclic compounds of cobalt (U.S. Pat. No. 4,179,359). The catalyst layers may also comprise the high surface area carbon (steam- or CO2 activated) itself, with no additional metal catalysts. The catalyst layers also comprise other non-catalytic components in addition to the catalyst material, usually polymeric materials which act as binders to hold the electrode layer together and may also perform an additional function in balancing the optimal hydrophobic or hydrophilic nature of the final structure.
These catalyst layers are usually formed into suitable mixtures of the components and deposited onto a suitable porous substrate, for example conducting carbon materials such as semi graphitised papers, cloths or foams, or particularly in the case of alkaline electrolyte systems, metal meshes such as nickel or stainless steel. These materials generally have a high bulk fibre density of greater than 0.4 g/cm3. The primary role of the substrate is to act as a physical support for the catalyst containing layers and to provide an electrically conducting structure. It also enables a mechanically stable electrode to be produced.
A major problem with conventional electrodes based on semi-solid porous carbon substrates is the lack of flexibility. The conventional electrodes are consequently easily damaged on handling which leads to high reject rates during manufactoring of the electrode. This obviously has an impact on cost. With conventional porous electrodes based on woven cloth substrates a problem concerns the lack of good dimensional stability, as the cloth can easily be stretched in the directions of the major planar faces (x and y directions). This can make the manufacturing of electrodes using these substrates very difficult and therefore costly.
The complexity of the new types of Polymer-Electrolyte Membrane (PEM) electrodes which operate in an acidic pH-range requires a number of expensive components and also accessories (like a compressor on the air side) which results in high costs. The cost per unit of these electrodes is far higher than is currently acceptable to make applications in power generation devices, such as fuel cells, commercially viable.